The present invention relates to a color cathode ray tube and particularly to a color cathode ray tube having an electron gun which houses a cathode structure in a cup-shaped first grid electrode and emits three in-line electron beams.
A cathode ray tube for use in an image display or a data terminal monitor comprises at least a vacuum envelope having a funnel with a faceplate having a phosphor screen on its inner surface, and a neck connected to the funnel and housing an electron gun structure which emits electron beams toward the phosphor screen.
FIG. 4 is a schematic sectional view for explaining the structure of a shadow mask type color cathode ray tube as an example of a color cathode ray tube to which the present invention is to be applied, and a reference numeral 20 denotes a faceplate, 21 denotes a neck, 22 denotes a funnel for connecting the faceplate to the neck, 23 denotes a phosphor screen constituting an image screen formed on the inner surface of the faceplate, 24 denotes a shadow mask, i.e., a color selection electrode, 25 denotes a mask frame forming a shadow mask assembly holding the shadow mask, 26 denotes an inner shield for shielding the color cathode ray tube from external magnetic fields, 27 denotes a suspension spring mechanism which suspends and supports the shadow mask assembly on studs heat-sealed to the inner side wall of the faceplate, 28 denotes an electron gun which emits 3 electron beams, Bs (.times.2) and Bc, 29 denotes a deflection device which deflects electron beams horizontally and vertically, 30 denotes an external magnetic correction device for performing color purity adjustment and centering correction, 31 denotes an internal conductive coating, 32 denotes stem pins through which various signals and operating voltages are supplied to the electron gun, 33 denotes an implosion protection tension band which holds the junction region of the panel and the funnel under tension, and 34 denotes a getter to obtain a high degree of vacuum within the vacuum envelope.
In the constitution as shown in FIG. 4, the vacuum envelope is comprised of the faceplate 20, the neck 21 and the funnel 22, and three electron beams, Bc and Bs.times.2, emitted in a line from the electron gun 28 are deflected in two directions of horizontal and vertical directions, by deflection magnetic fields generated by the deflection device 29 to scan the phosphor screen 23. Bc denotes a center beam and Bs denotes a side beam.
Three electron beams, Bs and Bs.times.2, are modulated respectively by three color signals, red (side beam Bs), green (center beam Bc) and blue (side beam Bs), supplied from the stem pins 32, and they are subjected to color selection in beam apertures in the shadow mask 24 disposed immediately in front of the phosphor screen 23 and reproduce a desired color image by impinging upon a red phosphor, a green phosphor and a blue phosphor of a mosaic three-color phosphor of the screen, respectively.
Electron beams are scanned over the whole phosphor screen 23 by horizontal and vertical deflection magnetic fields generated by the deflection device 29 on the way of movement from the electron gun 28 to the phosphor screen 23.
FIG. 5 is a side view for explaining a constitutional example of an electron gun to be used for the above-mentioned color cathode ray tube, wherein a reference numeral 1 denotes a cathode structure, 2 denotes a first grid electrode, 3 denotes a second grid electrode, 4 denotes a third grid electrode, 5 denotes a fourth grid electrode, 6 denotes a fifth grid electrode, 7 denotes a sixth grid electrode, 8 denotes a shield cup, 9 denotes an insulating rod, 32 denotes stem pins and 35 denotes a stem.
In FIG. 5, the first grid electrode 2 is a cup-shaped electrode, and the cathode structure 1 is housed within it.
The shield cup 8 is fixed on the sixth grid electrode 7, an anode, and the first grid electrode 2 and the second to sixth grid electrodes 3 to 7 are mounted in predetermined axially coaxially spaced relationship in a specified order on a pair of insulating supports 9 by tabs which are provided on the side wall of each of the electrodes and embedded in the insulating supports made of multiform glass.
The cathode structure 1 houses a heater coated with an insulating material, has a cathode cap having an electron emissive surface formed on its bottom supported by a sleeve fixed on a cathode support structure through an insulating plate of a ceramic material. The cathode support structure is inserted in the cup-shaped first grid electrode and it is welded to the first grid electrode at its open end.
As an example of disclosure of the prior art concerning an electron gun of this kind, Japanese Patent Laid-open No. Hei 7-161309 can be cited.
In the case of an electron gun of a cathode ray tube having a cathode structure incorporated in a cup-shaped first grid electrode, the spacing between an electron emission surface of a cathode of the cathode structure and the inner surface of the bottom of the first grid electrode can be established with high accuracy.
In the case of an electron gun having three cathode structures arranged in a line within the cup-shaped first grid electrode, during warm-up of a cathode ray tube, the spacing between the center beam aperture and a side beam aperture in the first grid electrode is varied by thermal expansion of the first grid electrode, and the first grid electrode is distorted to a dome-shape and the gap between the cup-shaped bottom surface formed with the electron beam apertures and the electron emissive surface of the cathode increases and also the spacing between the aperture for the center beam and that for the side beam becomes larger than a predetermined value; thereby static beam convergence drift occurs and also the time required for the beam current to reach a predetermined value becomes longer.
FIG. 6 is a schematic for explaining the thermal deformation of the cup-shaped first grid electrode, and only the first grid electrode is shown in a cross-sectional view and the cathode structure housed therein is omitted.
In FIG. 6, a reference numeral 2 denotes the first grid electrode, 2a denotes electron beam apertures, 2b denotes metal tabs butt-welded to the first grid electrode 2 and 9 denotes the insulating rods.
In a case where the first grid electrode 2 is a cup-shaped one, metal tabs 2b for supporting the first grid electrode 2 and embedded in the insulating supporting rods are made of the same material as the first grid electrode, which is generally 42% Ni--Fe.
When the heater of the cathode structure is energized, the cathode temperature is raised and the first grid electrode 2 is thermally expanded by the radiant heat from the cathode, the tabs 2b are also thermally expanded and extends in the direction of the arrows A and the first grid electrode 2 expands in the direction of an arrow B; thereby the spacing between the electron emissive surface of the cathode structure and the bottom of the first grid electrode is made larger. When the spacing between the electron emissive surface and the bottom of the first grid electrode 2 is made larger, the cutoff voltage is made lower; thereby the quantity of electron beams drawn from the cathode is lowered and the rising speed of screen brightness is made slow.
As mentioned above, in the case of a conventional cathode structure housed within a cup-shaped first grid electrode, there has been problems that the rising speed of screen brightness is slow during warm-up, a long period of time is needed till it reaches a stable state, and also the amount of static beam convergence drift is large.
FIG. 7 is an illustration of a beam current build-up characteristic during warm-up of a color cathode ray tube employing a prior art electron gun.
As shown in FIG. 7, in the case of a conventional cathode ray tube, a build-up curve 10a of a beam current has a gentle slope which shows the behavior of a beam current until it reaches a specified value 10 after the cathode ray tube is turned on.